A permanent-magnet holding plate or mandrel is known for use in a machine shop to hold a workpiece while it is being machined. Such a holding plate can be set up so that it can be permanently magnetized and so that it can also be thoroughly demagnetized. Thus the holder is magnetized before or after a workpiece is positioned on it, and after the machining operation it is demagnetized to allow the finished workpiece to be separated from the holder.
More specifically such a magnet plate has high-induction magnets with a weak coercive field, typically aluminum-nickel-cobalt compounds that are surrounded by electrical coils and capped by ferrous pole pieces. Such a plate is activated so that it can magnetically hold a workpiece by passing an electrical current through the coils to permanently magnetize the magnets. During the machining operation the permanent magnetization serves to hold the workpiece in place. Once the machining operation is completed a current is passed through the coils so as to demagnetize the magnets, allowing the workpiece to be separated from the holder.
A first type of such electropermanent magnetic plate has the array of magnets covered by a pole plate and each magnet is surrounded by a respective coil of enamel-coated copper wire of very fine gauge and having a great number of turns. It takes between 0.8 sec and 1.0 sec of energization of the coils by direct current of a predetermined polarity to magnetize the magnets. Demagnetization is effected by passing about a dozen pulses of oppositely polarized current through the magnets over about 10 sec.
Such a system is very expensive, principally because in addition to the pole plate which costs about 30% of the total cost of the unit the coils are expensive. In addition here the bobbins are sealed underneath the pole plate.
In another known type of such a permanent-magnet holder each magnet is surrounded by a simple coil comprising only a few--six to ten --turns of copper wire with PVC insulation, the individual wires having a section between 1.5 and 6 mm.sup.2. Such magnets are magnetized and discharged by currents obtained by discharging banks of condensers in about 3 to 5 millisec at a current of about 500 amp which is about the maximum one can get from the condensers.
While this type of magnetic holder is itself fairly cheap to manufacture, the supply circuit is relatively expensive principally because of the condensers. In addition such an arrangement is slow, taking 20 to 30 sec to demagnetize the holder, a time that is unacceptably long in today's rapid industrial processes.
It has been suggested to try to combine the best of these two types of holders by eliminating the pole plate in a system with fine-wire coils. The coils are potted in epoxy to protect them. The power supply for such a system is simple and inexpensive, and magnetization and demagnetization are relatively quick.
More particularly in this hybrid system the feed circuit comprises thyristors which are fed alternating line current and which are tripped so that they feed to the magnets three pulses of the same sign for magnetization. For demagnetization the thyristors are set to operate degressively and send positive and negative pulses through the coils.
While the feed circuit is fairly simple, making the holding plate is quite difficult. The fine-wire coils are expensive to produce as is the potting since the surfaces of the pieces must be abraded so the potting will adhere. The potting itself is normally done under vacuum so it is an expensive procedure. In addition several feed circuits must be provided for a large-area holding plate.